The present invention relates to an ultrasound diagnosing system for providing an ultrasound cross-sectional image of a living subject using a pulse-echo method, and a two-dimensional distribution image of blood flow information corresponding to the cross-sectional image using a Doppler method, thereby forming a two-dimensional image in which the blood flow information image is superposed on the cross-sectional image.
In a conventional composite system of an ultrasound blood flow measuring apparatus using the Doppler method and an electronic scanning ultrasound imaging apparatus using the pulse-echo method, a blood flow speed at any point of the cross-sectional image can be observed. However, the blood distribution in, e.g., the heart, cannot be one- or two-dimensionally observed.
A system for forming a visible two-dimensional image of blood flow information of a living subject by utilizing a moving-target indicator (MTI) technique used in radar applications is described by K. Namekawa et al., "REALTIME TWO-DIMENSIONAL BLOOD FLOW IMAGING SYSTEM", pp. 541-542; Y. Seo et al., "Real-Time Ultrasonic Blood Flow Imaging System", pp. 543-544; and so on.
For example, Japanese Patent Disclosure (Kokai) No. 58-188433 describes a composite system of a cross-sectional imaging apparatus and a blood flow imaging apparatus for forming a two-dimensional image of blood flow information. A typical example of the composite system will be described with reference to FIG. 1. Referring to FIG. 1, an electronic sector-scanning type B-mode imaging apparatus is employed as a cross-sectional imaging apparatus, in combination with the above blood flow imaging apparatus.
Electronic sector-scanning type ultrasound transducer 1 is brought into contact with the skin of a living subject under examination, e.g., the skin near the heart. Ultrasound pulse beams are emitted from transducer 1 in response to drive pulses supplied from electronic sector-scanning type B-mode imaging section 2 (to be referred to as a B-mode section hereinafter). Ultrasound waves, i.e., ultrasound echoes reflected by organic tissue such as the heart muscle or by blood cells are received by transducer 1 and converted thereby to a reception signal, i.e., an electrical signal. The reception signal is supplied to B-mode section 2 and processed thereby according to known techniques to obtain an echo video signal for displaying an ultrasound cross-sectional image.
The reception signal supplied to B-mode section 2 is also transferred without any processing to first and second mixers 4a and 4b in blood imaging section 3. Mixer 4a receives a sinusoidal wave of frequency f0 generated by reference signal generator 5 and shifted 90.degree. by 90.degree. phase shifter 6, as well as the reception signal. Frequency f0 corresponds to the center frequency of the ultrasound wave used in this system. Mixer 4b receives a sinusoidal wave (this wave is not phase-shifted) generated by generator 5, and the reception signal. Mixers 4a and 4b multiply the respective sinusoidal waves with the reception signals from B-mode section 2. This multiplication corresponds to phase detection, using each sinusoidal wave as a reference signal. In other words, Doppler shifts, i.e., the phase shifts of the reception echo components with respect to the sinusoidal waves are detected. In this case, the the phases of the reception echo components are detected by mixers 4a and 4b using the 90.degree.-shifted signal and the signal from generator 5, respectively, using, i.e., a quadrature detection technique.
The echo signals from the living subject upon radiation of the ultrasound pulse beams are signals the time base of which corresponds to the depth in the subject's body. More specifically, the phase detection result of the reception signal for each ultrasound pulse represents that the Doppler shift (motion along the ultrasound beam direction) values at different depths along the ultrasound beam direction are plotted along the time base. In this case, in order to detect a change in Doppler shift as a function of time, ultrasound pulses are emitted a plurality of times at a plurality of beam positions during sector scanning, and the reception signals corresponding to the pulses are sequentially processed.
Mixers 4a and 4b output Doppler signals which are 90.degree. out of phase. These Doppler signals are supplied to first and second A/D (analog-digital) converters 7a and 7b, and are converted to digital signals thereby. The resultant digital signals represent a plurality of different points along the ultrasound beam direction at sampling timings of converters 7a and 7b, and are supplied to first and second MTI filters 8a and 8b by converters 7a and 7b.
Filters 8a and 8b eliminate low-frequency components as unnecessary signal components (so-called clutter components), i.e., the echo signal components from stationary and low-motion portions (relative to blood flow) such as a heart wall. Each MTI filter has a plurality of line memories (the number of which corresponds to the repetition frequency of the pulses at the same beam position) for storing data of one-scanning line, i.e., one-ultrasound pulse data. Each MTI is constituted by a digital filter responsive to a change in data as a function of time for each pixel, on the basis of data corresponding to identical pixels in the plurality of line memories.
Output signals from filters 8a and 8b are supplied to calculating circuit 9. Circuit 9 processes the output signals from filters 8a and 8b according to correlation processing (to be described later) and calculates average speed V of a blood flow speed.
Changes in data values (i.e., data values corresponding to a specific point or pixel) from converter 7a are illustrated in FIG. 2A, and changes in data values from converter 7b are illustrated in FIG. 2B.
As is apparent from FIGS. 2A and 2B, these curves include echo components x.sub.1 -x.sub.2 - . . . -x.sub.i - . . . and y.sub.1 -y.sub.2 - . . . -y.sub.i - . . . , and clutter components (low-frequency variations represented by broken lines in FIGS. 2A and 2B). As previously stated, the variations in outputs from converters 7a and 7b have a 90.degree. phase difference. Output variations X.sub.a (t) and X.sub.b (t) of converters 7a and 7b are given as follows: ##EQU1##
The first terms of the right-hand sides of equations (1) and (2) represent the clutter components, and the second terms thereof represent the echo components: .omega..sub.d &lt;&lt;.omega..sub.c.
In ideal MTI filters, the low-frequency components, i.e., the clutter components, can be completely eliminated. In that case, equations (1) and (2) can be rewritten to include only the second terms: ##EQU2##
Circuit 9 calculates average frequency .omega..sub.d using X.sub.a (t) and X.sub.b (t) of equations (3) and (4). This calculation is performed by a high-speed digital calculation circuit (i.e., a combination of operation circuits for constituting high-speed hardware) using an autocorrelation method.
Frequency .omega..sub.d is calculated by autocorrelation as follows: EQU .omega..sub.d =-j{C'.sub.(0) C.sub.(0) }=.intg..omega.S(.omega.)d.omega./.intg.S(.omega.)d.omega.(5)
where C.sub.(0) is the autocorrelation function C.sub.(.tau.) at .tau.= 0, C'.sub.(0) is the derivative of C.sub.(.tau.) at .tau.=0.
In the case of discrete X.sub.a (t) and X.sub.b (t), as shown in FIGS. 2A and 2B, equations (1) and (2) can be substituted by equations (1') and (2'): EQU X.sub.a =(x.sub.1,x.sub.2, . . . , x.sub.n, x.sub.n+1) (1') EQU X.sub.b =(y.sub.1, y.sub.2, . . . , y.sub.n, y.sub.n+1) (2')
In this case, frequency .omega..sub.d can be calculated as follows: ##EQU3##
A detailed arrangement of circuit 9 for performing equation (6) will be described with reference to FIG. 3.
Output data x.sub.i and output data y.sub.i from filters 8a and 8b are stored in memories 10a and 10b in units of pixels. Two of data x.sub.i and y.sub.i stored in memories 10a and 10b, and data x.sub.i+1 and y.sub.i+1 succeeding data x.sub.i and y.sub.i by one sampling are selected by first and second switching circuits 11 and 12. Multiplier 13 performs calculations x.sub.i x.sub.i+1, y.sub.i y.sub.i+1, x.sub.i y.sub.i+1, and y.sub.i x.sub.i+1. The products are stored in third to sixth memories 14 to 17 in units of pixels. First adder 18 performs .SIGMA.(y.sub.i x.sub.i+1 -x.sub.i y.sub.i+1) using storage data in memories 14 and 15. Second adder 19 performs .SIGMA.(x.sub.i x.sub.i+1 +y.sub.i y.sub.i+1) using storage data in memories 16 and 17. Divider 20 performs .SIGMA.(y.sub.i x.sub.i+1 -x.sub.i y.sub.i+1)/.SIGMA.(x.sub.i x.sub.i+1 +y.sub.i+1) using outputs from adders 18 and 19. The calculations of adders 18 and 19 and divider 20 are performed in units of pixels. Data converter 21, using a ROM (read-only memory) for storing conversion table data, performs data conversion in accordance with the calculation of equation (6) for calculating frequency .omega..sub.d by an arc tangent of the quotient from divider 20, and with the calculation for obtaining average speed V from frequency .omega..sub.d, thereby obtaining speed V: EQU V=C.omega..sub.d /4.pi.f.sub.0 ( 7)
where f.sub.0 is the center frequency of the ultrasound beam, and C is the sonic speed.
The operation result from circuit 9 is output in units of pixels constituting the ultrasound cross-sectional image (actual output data is two-dimensional data of 128.times.64 dots).
Output data from circuit 9 is supplied to D.S.C. (digital scan converter) 22 of FIG. 1. D.S.C. 22 also receives a digital signal from A/D converter 23 for A/D converting the echo video signal from B-mode section 2. D.S.C. 22 converts the two-dimensional data received from section 3 and circuit 9 to color data, in a color corresponding to speed V, and the color data is superposed on the echo video signal supplied from B-mode section 2 through converter 23. The scanning scheme of the two-dimensional data is thus changed from ultrasound scanning to TV scanning. An output from D.S.C. 22 is supplied to color processor 24, and is processed thereby in accordance with the color data. An output from processor 24 is supplied to display 26 through D/A (digital-analog) converter 25. Only a blood flow portion in the black-and-white B-mode image is displayed in a color corresponding to the blood flow information (i.e., speed V). The correspondence between the blood flow information and the display colors is predetermined.
In the conventional system, in order to calculate speed V of a two-dimensional image of 128.times.64 dots (pixels) in a real-time mode, the number of data outputs (samples) per one dot is limited to, e.g., 8, due to sonic speed limitations. The number of data outputs is thus limited, and the clutter components cannot be sufficiently decreased due to filter transient response. Therefore, the conventional systems cannot provide accurate average blood speed information.